High-performance ceramic materials are essential for many engineering applications. Ultrafine ceramic powders are needed to provide ceramic structures having the integrity, strength, and uniformity necessary to meet high performance requirements. To obtain many of the desirable properties associated with advanced ceramics, e.g., toughness, high ductility, low sintering temperature and/or superplasticity, ceramic powders having nanometer scale crystalline grain structure, uniformity of crystalline phase, limited degree of particle aggregation, chemical purity, and narrow particle and grain size distributions are essential. Bowen, (1980) Mater. Sci. Eng., 44:1; Andres et al, (1989) J. Mater. Res., 4(3):704; Wakai and Nagano, (1991) J. Mater. Sci., 26(1):241. Large scale exploration of the properties of these materials has been limited, however, by a lack of large quantities of inexpensive nanophase powder having the desired properties.
To address this need, researchers have been exploring many methods of nanophase powder production. For example, spray drying or spray pyrolysis has been used to produce unagglomerated crystalline powders. In this process, precursor salts are dissolved in water which is sprayed into a high temperature environment. Ceramic particles form as a result of nucleation in the liquid phase, driven by evaporation of water. Although chemical purity is high, processing rates are limited by the need to maintain very low concentrations of precursor solute in the droplets to prevent formation of undesirable fragments and cenospheres. Kodas et al., (1988) Appl. Phys. Lett., 52:1622; Zhang et al., (1990) J. Am. Ceram. Soc., 73(1):61. Industrial flame processes such as SiCl.sub.4 oxidation are inherently high rate, but are best suited to single component systems such as TiO.sub.2 or SiO.sub.2, and may produce large sintered agglomerated powders due to the extended residence time at high temperature. Ulrich and Riehl, (1982) J. Colloid Inter. Sci., 87:257. Plasma and laser processes offer improved control over the system chemistry, but particles produced by these processes exhibit extensive agglomeration. Rice, (1986) J. Am. Ceram Soc., 69:C-185; Ono et al., (1987) Plasma Chem. Plasma Proc., 7:207. Sol-gel processes also offer precise control over powder chemistry, but produce agglomerated powders which shrink extensively during the lengthy, energy-intensive, high temperature calcining step which follows. Condensation and oxidation of a metal deposited on a cold finger in a vacuum ("gas condensation method") has been used to generate high purity titania. Siegel et. al., (1988) J. Mater. Res., 3:1367. However, this batch process is inherently low rate.
While the processes described above meet some of the criteria for optimum production of nanophase powders, none combine the high temperatures required for crystallinity with the short processing times required for minimal agglomeration.